MDO multilayer film

ABSTRACT

A multilayer film having Machine Direction Orientation (MDO) is prepared by first co-extruding a multilayer film, then stretching the multilayer film in the machine direction at a temperature lower than the melting point of the polyethylene resin that is used to prepare the film. At least one layer of the film is a first polyethylene composition having a density of from about 0.94 to about 0.97 g/cc and at least one second layer is prepared from a polyethylene composition having a lower density than the first polyethylene composition. This disclosure enables the manufacture of films having outstanding barrier properties (low Water Vapor Transmission Rate, WVTR, and low Oxygen Transmission Rate, OTR) and good physical properties.

TECHNICAL FIELD

This disclosure relates to Machine Direction Oriented (MDO) filmsprepared from a co-extruded multilayer polyethylene precursor film whichincludes a layer made from a High Density Polyethylene (“HDPE”)composition.

BACKGROUND ART

Polyethylene film is used in a wide variety of applications such as thepreparation of food packages, heavy duty sacks, collation shrinkpackaging and trash can liners. Polyethylene is commonly described interms of melt index (which provides an indication of the molecularweight of the polyethylene) and density.

The MDO films described in this disclosure are prepared with a precursorfilm (i.e., a non-stretched film) that contains at least one layer ofHDPE and at least one layer that is made from a polyethylene having alower density.

HDPE is now commonly used in the preparation of “barrier film” which isresistant to the transmission of water. Barrier film is especiallysuitable for packaging dry foods such as breakfast cereals and crackers.Monolayer HDPE barrier film has limited use because several of theproperties of such film—including impact strength, tear strength andsealing temperature—are inferior to those of lower density polyethylene.The use of a multilayer film that contains a layer of HDPE and a layerof a lower density resin can mitigate some of these problems but this isgenerally done at a cost of reducing the barrier properties of themultilayer film in comparison to a monolayer HDPE film.

This disclosure provides a multilayer film that is subjected to machinedirection orientation (MDO) in a process whereby the film is stretchedafter the film is initially formed in a conventional extrusion process.One layer of the film is made from a high density polyethylenecomposition which, in one embodiment, contains a nucleating agent. Themultilayer MDO films of this disclosure exhibit a balance of barrier andphysical properties that makes them suitable for many packagingapplications.

DISCLOSURE OF INVENTION

In one embodiment, this disclosure provides a method for producing anoriented multilayer film comprising:

co-extruding a multilayer film comprising

a first layer prepared from a high density polyethylene compositionhaving a melt index, I₂, of from about 0.2 to about 10 grams per 10minutes, a density of from about 0.95 to about 0.97 g/cc and from about100 to about 9000 parts per million of a nucleating agent; and

a second layer prepared from a second polyethylene composition having amelt index, I₂, of from about 0.2 to about 10 grams per 10 minutes and adensity which is lower than the density of said first polyethylenecomposition by an amount of from about 0.010 to about 0.060 g/cc; and

stretching said multilayer film in the machine direction at a stretchratio of from about 1:2 to about 1:12.

In another embodiment, this disclosure provides a method for producingan oriented multilayer film comprising:

co-extruding a multilayer film comprising

a first layer prepared from a high density polyethylene compositionhaving a melt index, I₂, of from about 0.2 to about 10 grams per 10minutes; a density of from about 0.95 to about 0.97 g/cc and a Mw/Mn offrom about 5 to about 12, with a first proviso that said firstpolyethylene composition is a blend of at least two blend componentscomprising a first blend component having a Mw/Mn of from about 2 toabout 4 and a second blend component having a Mw/Mn of from about 2 toabout 4; with a second proviso that the melt index, I₂, of the secondblend component is at least ten times greater than the melt index of thefirst blend component; and

a second layer prepared from a second polyethylene composition having amelt index, I₂, of from about 0.2 to about 10 grams per 10 minutes and adensity which is lower than the density of said first polyethylenecomposition by an amount of from about 0.010 to about 0.060 g/cc; and

stretching said multilayer film in the machine direction at a stretchratio of from about 2/1 to about 12/1.

As noted above, high density polyethylene provides excellent barrierproperties but inferior strength and tear properties in comparison tolower density polyethylenes. A multilayer film which contains a layer ofHDPE and a layer of lower density polyethylene can provide a film havingbetter physical properties (but at the expense of lower barrierproperties for a given thickness of film). The present disclosureenables the manufacture of a multilayer film that includes a layer ofHDPE and a layer of a lower density polyethylene but still providesexcellent barrier performance.

BEST MODE FOR CARRYING OUT THE INVENTION Polyethylene

The MDO films of this disclosure are prepared from a multilayer film inwhich at least one of the layers is prepared from an HDPE composition(described in Part A.1, below) and at least one of the layers isprepared from a polyethylene composition having a lower density than theHDPE composition (described in Part A.2, below).

A.1 HDPE Composition

HDPE is a common item of commerce. Most commercially available HDPE isprepared from a catalyst that contains at least one metal; non-limitingexamples include chromium, titanium, zirconium and hafnium. HDPE that ismade from a Cr catalyst typically contains some long chain branching(LCB).

HDPE that is made from a group IV metal (titanium, zirconium andhafnium) generally contains less LCB than HDPE made from a Cr catalyst.While not wishing to be bound by theory, it has been postulated that thepresence of LCB can reduce the effectiveness of a nucleating agent. Theuse of HDPE prepared with a group IV metal (especially Ti or Zr) isgenerally preferred for use in this disclosure.

As used herein, the term “HDPE” refers to a polyethylene (orpolyethylene blend composition, as required by context) having a densityof from about 0.95 to about 0.97 grams per cubic centimeter (g/cc). Inan embodiment, the melt index (“I₂”) of the HDPE is from about 0.2 toabout 10 grams per 10 minutes.

In an embodiment, the HDPE is provided as a blend of two HDPEs havingmelt indices that are separated by at least a decade. Further details ofthis HDPE blend composition follow.

HDPE Blend Composition Blend Components Blend Component a)

Blend component a) of the polyethylene composition used in thisembodiment comprises an HDPE with a comparatively high melt index. Asused herein, the term “melt index” is meant to refer to the valueobtained by ASTM D 1238 (when conducted at 190° C., using a 2.16 kgweight). This term is also referenced to herein as “I₂” (expressed ingrams of polyethylene which flow during the 10 minute testing period, or“gram/10 minutes”). As will be recognized by those skilled in the art,melt index, I₂, is inversely proportional to molecular weight. In oneembodiment, blend component a) has a comparatively high melt index (or,alternatively stated, a comparatively low molecular weight) incomparison to blend component b).

The absolute value of I₂ for blend component a) in these blends isgenerally greater than 5 grams/10 minutes. However, the “relative value”of I₂ for blend component a) is more important and it should generallybe at least 10 times higher than the I₂ value for blend component b)[which I₂ value for blend component b) is referred to herein as I₂′].Thus, for the purpose of illustration: if the I₂′ value of blendcomponent b) is 1 gram/10 minutes, then the I₂ value of blend componenta) is preferably at least 10 grams/10 minutes.

In one embodiment, blend component a) may be further characterized by:

having a density of from about 0.95 to about 0.97 g/cc; and

being present in an amount of from about 5 to about 60 weight % of thetotal HDPE blend composition (with blend component b) forming thebalance of the total composition) with amounts of from about 10 to about40 weight %, especially from about 20 to about 40 weight %, beinggenerally preferred. It is permissible to use more than one high densitypolyethylene to form blend component a).

The molecular weight distribution [which is determined by dividing theweight average molecular weight (Mw) by number average molecular weight(Mn) where Mw and Mn are determined by gel permeation chromatography,according to ASTM D 6474-99] of component a) is preferably from about 2to about 20, especially from about 2 to about 4. While not wishing to bebound by theory, it is believed that a low Mw/Mn value (from 2 to 4) forcomponent a) may improve the crystallization rate and overall barrierperformance of blown films prepared according to the disclosed process.

Blend Component b)

Blend component b) is also a high density polyethylene which has adensity of from about 0.95 to about 0.97 g/cc (preferably from about0.955 to about 0.968 g/cc).

The melt index of blend component b) is also determined by ASTM D 1238at 190° C. using a 2.16 kg load. The melt index value for blendcomponent b) (referred to herein as I₂′) is lower than that of blendcomponent a), indicating that blend component b) has a comparativelyhigher molecular weight. The absolute value of I₂′ is preferably fromabout 0.1 to about 2 grams/10 minutes.

The molecular weight distribution (Mw/Mn) of component b) is notcritical to the success of the embodiments described in this disclosure,though a Mw/Mn of from about 2 to about 4 is preferred for component b).

Finally, the ratio of the melt index of component b) divided by the meltindex of component a) is preferably greater than 10/1.

Blend component b) may also contain more than one HDPE resin.

Overall HDPE Blend Composition

The overall high density blend composition is formed by blendingtogether blend component a) with blend component b). In an embodiment,this overall HDPE composition has a melt index (ASTM D 1238, measured at190° C. with a 2.16 kg load) of from about 0.5 to about 10 grams/10minutes (preferably from about 0.8 to about 8 grams/10 minutes).

The blends may be made by any blending process, such as: 1) physicalblending of particulate resin; 2) co-feed of different HDPE resins to acommon extruder; 3) melt mixing (in any conventional polymer mixingapparatus); 4) solution blending; or, 5) a polymerization process whichemploys 2 or more reactors.

A suitable HDPE blend composition may be prepared by melt blending thefollowing two blend components in an extruder:

from about 10 to about 30 weight % of component a): where component a)is an HDPE resin having a melt index, I₂, of from about 15 to about 30grams/10 minutes and a density of from about 0.95 to about 0.97 g/ccwith

from about 90 to about 70 weight % of component b): where component b)is an HDPE resin having a melt index, I₂, of from about 0.8 to about 2grams/10 minutes and a density of from about 0.95 to about 0.97 g/cc.

An example of a commercially available HDPE resin which is suitable forcomponent a) is sold under the trademark SCLAIR™ 79F, which is an HDPEresin that is prepared by the homopolymerization of ethylene with aconventional Ziegler Natta catalyst. It has a typical melt index of 18grams/10 minutes and a typical density of 0.963 g/cc and a typicalmolecular weight distribution of about 2.7.

Examples of commercially available HDPE resins which are suitable forblend component b) include (with typical melt index and density valuesshown in brackets):

SCLAIR™ 19G (melt index=1.2 grams/10 minutes, density=0.962 g/cc);

MARFLEX™ 9659 (available from Chevron Phillips, melt index=1 grams/10minutes, density=0.962 g/cc); and

ALATHON™ L 5885 (available from Equistar, melt index=0.9 grams/10minutes, density=0.958 g/cc).

A preferred HDPE blend composition is prepared by a solutionpolymerization process using two reactors that operate under differentpolymerization conditions. This provides a uniform, in situ blend of theHDPE blend components. An example of this process is described in U.S.Pat. No. 7,737,220 (Swabey et al.). As shown in the examples, this HDPEcomposition provides good barrier performance.

A.2 Lower Density Polyethylene

The precursor multilayer film used in this disclosure contains at leastone layer that is prepared from a polyethylene composition having alower density than the HDPE composition described above.

In an embodiment, this lower density composition is a medium densitypolyethylene composition having a density of from about 0.925 to about0.945 g/cc.

In another embodiment, this lower density composition is a linear lowdensity polyethylene composition having a density of from about 0.900 toabout 0.925 g/cc.

In another embodiment, the melt index, I₂, of the lower densitycomposition is from about 0.2 to about 10 g/10 minutes especially fromabout 0.5 to about 5 g/10 minutes.

In another embodiment, the lower density composition is prepared with asingle site catalyst (as shown in examples).

It will be recognized by those skilled in the art that lower densitypolyethylene generally has better strength properties (i.e., betterimpact strength, puncture resistance and tear strength) than higherdensity polyethylene. In contrast, higher density polyethylene typicallyproduces films having better barrier properties than films prepared fromlower density polyethylene, e.g., LLDPE. The superior WVTR of monolayerHDPE films (in comparison to LLDPE films) is shown in the examples.

The examples also illustrate that multilayer films have a MVTR that isproportional to the MVTR of the individual layers (in the unstretchedstate). However, the examples also show that the present disclosureenables the manufacture of a multilayer MDO film that has superior MVTRthan an unstretched HDPE film of the same thickness.

B. Nucleating Agents

The term nucleating agent, as used herein, is meant to convey itsconventional meaning to those skilled in the art of preparing nucleatedpolyolefin compositions, namely an additive that changes thecrystallization behavior of a polymer as the polymer melt is cooled. Inan embodiment, the nucleating agents are “organic” (i.e., componentswhich contain carbon and hydrogen atoms) as opposed to inorganicnucleating agents such as talc or zinc oxide (which in general, are muchless effective nucleating agents than the “organic” nucleating agentsdescribed above).

A review of nucleating agents is provided in U.S. Pat. Nos. 5,981,636;6,465,551 and 6,599,971.

Examples of conventional nucleating agents which are commerciallyavailable and in widespread use as polypropylene additives are thedibenzylidene sorbital esters (such as the products sold under thetrademark Millad™ 3988 by Milliken Chemical and Irgaclear™ by CibaSpecialty Chemicals).

The nucleating agents should be well dispersed in the HDPE. The amountof nucleating agent used is comparatively small, e.g., from 100 to 9000parts by million per weight (based on the weight of the HDPE) so it willbe appreciated by those skilled in the art that some care must be takento ensure that the nucleating agent is well dispersed. It is preferredto add the nucleating agent in finely divided form (i.e., having anaverage particle size of less than 50 microns, especially less than 10microns) to the polyethylene to facilitate mixing. The use of a“masterbatch” of the nucleator (where the term “masterbatch” refers tothe practice of first melt mixing the nucleating agent with a smallamount of polyethylene—then melt mixing the “masterbatch” with theremaining bulk of the HDPE resin) can also help to disperse thenucleating agent.

Examples of nucleating agents which may be suitable for use in thepresent disclosure include the cyclic organic structures disclosed inU.S. Pat. No. 5,981,636 (and salts thereof, such as disodium bicyclo[2.2.1] heptene dicarboxylate); the saturated versions of the structuresdisclosed in U.S. Pat. No. 5,981,636 (as disclosed in U.S. Pat. No.6,465,551; Zhao et al., to Milliken); the salts of certain cyclicdicarboxylic acids having a hexahydrophtalic acid structure (or “HHPA”structure) as disclosed in U.S. Pat. No. 6,559,971 (Dotson et al., toMilliken); phosphate esters, such as those disclosed in U.S. Pat. No.5,342,868 and those sold under the trade names NA-11 and NA-21 by AsahiDenka Kogyo and metal salts of glycerol (especially zinc glycerolate).The accompanying examples illustrate that the calcium salt ofI,2-cyclohexanedicarboxylic acid, calcium salt (CAS registry number491589-22-1) provides exceptionally good results. The use of more thanone nucleating agent may also be suitable. For example, talc and zincoxide are commonly used as additives in polyethylene resin (because theyprovide “anti-blocking” and “acid scavenging” performance, respectively)but they are also known to provide some nucleation performance. Acomposition which contains at least one of the organic nucleating agentsdescribed above plus talc (and/or zinc oxide) is suitable for use inthis disclosure.

C. MDO Description

Machine Direction Orientation (MDO) is well known to those skilled inthe art and the process is widely described in the literature. MDO takesplace after a film has been formed. The “precursor” film (i.e., the filmas it exists prior to the MDO process) may be formed in any conventionalfilm molding process. Two film forming processes that are in widecommercial use (and are suitable for preparing the precursor film) arethe blown film process and the cast film process.

The precursor film is stretched (or, alternatively stated, strained) inthe MDO process. The stretching is predominantly in one direction, whichis the “machine direction” from the initial film molding process (i.e.,as opposed to the transverse direction. The thickness of the filmdecreases with stretching. A precursor film that has an initialthickness of 10 mils and a final thickness after stretching of 1 mil isdescribed as having a “stretch ratio” or “draw down” ratio of 1:10.

In general, the precursor film may be heated during the MDO process. Thetemperature is typically higher than the glass transition temperature ofthe polyethylene and lower than the melting temperature and morespecifically, is typically from about 70 to about 120° C. for apolyethylene film. Heating rollers are generally used to provide thisheat.

A typical MDO process utilizes a series of rollers that operate atdifferent speeds to apply a stretching force on a film. In addition, twoor more rollers may cooperate together to apply a comparison force (or“nip”) on the film.

The stretched film is generally overheated (i.e., maintained at anelevated temperature—typically, from about 90 to about 125° C.) to allowthe stretched film to relax.

The disclosure is illustrated in further detail by the following nonlimiting example.

EXAMPLE

The following test methods were used.

Melt Index: “I₂”, was determined according to ASTM D1238. [Note: I₂measurements are made with a 2.16 kg weight at 190° C.] Test results arereported in units of grams/10 minutes.

Number average molecular weight (Mn), weight average molecular weight(Mw) and MWD (calculated by Mw/Mn) were determined by high temperatureGel Permeation Chromatography “GPC” with differential refractive index“DRI” detection using universal calibration.

Secant Modulus (MD/TD) was determined according to ASTM D882.

Density was determined using the displacement method according to ASTMD792.

Gloss was determined by ASTM D2457.

Haze was determined by ASTM D1003.

Water Vapor Transmission Rate (“WVTR”, expressed as grams of water vaportransmitted per 100 square inches of film per day at a specified filmthickness (mils), or g/100 in²/day) was measured in accordance with ASTMF1249-90 with a MOCON permatron developed by Modern Controls Inc. atconditions of 100° F. (37.8° C.) and 100% relative humidity.

The following polyethylene compositions were used in this example.

sHDPE-1 is an ethylene homopolymer composition that is prepared in adual reactor solution polymerization process using a single sitecatalyst, using procedures in substantial accordance with thosedescribed in U.S. Pat. No. 7,737,220. A high molecular weighthomopolymer (having an Mw/Mn of about 2) is prepared in the firstreactor and a low molecular weight homopolymer is prepared in the secondreactor. The I₂ of the low molecular weight homopolymer from reactor 2is more than ten times higher than the I₂ of the high molecular weighthomopolymer from reactor 1. The density of the overall blend compositionis about 0.967 g/cc, the melt index (I₂) is about 1.2 g/10 minutes andthe Mw/Mn is about 8.

sHDPE-1 also contains about 1200 ppm of a nucleating agent (sold underthe trade name HYPERFORM® HPN20E by Milliken Chemicals).

HDPE-2 is an ethylene homopolymer made with a Ziegler Natta catalyst(containing Ti). It has a density of about 0.96 g/cc and a melt index ofabout 1.2 [19G].

HDPE-3 is an ethylene homopolymer made with a Ziegler Natta catalyst(containing Ti). It has a density of about 0.96 g/cc and a melt index ofabout 1 (19C).

sLLDPE-1 is an ethylene-octene copolymer made with a single sitecatalyst (containing Ti). It has a density of about 0.916 g/cc, a meltindex of about 0.7 g/cc, a CDBI of greater than 70%, an Mw/Mn of about2.8 and was prepared in a dual reactor solution polymerization process.

For convenience, some of the above described properties are provided inTable 1.

TABLE 1 Polyethylene Composition Type Melt Index, dg/min Density, g/ccsHDPE-1 1.2 0.967 HDPE-2 1.2 0.96 HDPE-3 1 0.96 sLLDPE 0.7 0.916

Three layer films were then prepared on a blown film line. Forconvenience, the three layers may be referred to as A/B/C—with layers Aand C being the external layers (often referred to as “skin” layers) andlayer B being the core layer. Table 2 shows the composition andthickness of the films.

These thick films were then subjected to a Machine Direction Orientation(MDO) process. The MDO was done at temperatures between about 98 toabout 121° C. at the stretch ratios shown in Table 3. By way of example,(and for clarity), the precursor film 1 from Table 2 was used to prepare3 MDO films (stretch ratios 1:6; 1:8; and 1:9) as shown in Table 3.

Table 3 shows that various thinner films were prepared by stretching thethick film by increasing amounts.

The thickness and the Water Vapor Transmission rate of these films arereported in Table 3. As shown in Table 3, comparative monolayer filmsprepared from only the nucleated HDPE composition (sHDPE-1) haveexcellent barrier properties (as indicated by low WVTR values) andcomparative films prepared from only the sLLDPE composition havecomparatively poor barrier properties.

Precursor films comprising two skin layers of sLLDPE and a core layer ofsHDPE had intermediate barrier properties. However, when these precursorfilms were subjected to the MDO process at stretch ratios of at least1:6, a very surprising and unexpected observation was made—the absoluteWVTR values of these films is better than the WVTR value of the filmprepared from sHDPE-1 alone.

TABLE 2 Film Layer A (wt %) Layer B (wt %) Layer C (wt %) 1 sHDPE-1sHDPE-1 sHDPE-1 (25) (50) (25) 2 HDPE-2 HDPE-2 HDPE-2 (25) (50) (25) 3sLLLDPE-1 sHDPE-1 sLLDPE-1 (25) (50) (25) 4 sLLDPE-1 sHDPE-1 sLLDPE-1(15) (70) (15) 5 sLLDPE-1 HDPE-3 sLLDPE-1 (25) (50) (25)

TABLE 3 % change Film WVTR against non- Gauge G/100 MDO film FilmStretch Ratio mil In² · mil/D (1:1) 1 Stretch Ratio 1:1 7.2 0.1728 0.01.6 Stretch Ratio 1:6 1.7 0.2329 34.8 1.8 Stretch Ratio 1:8 1.1 0.2049318.6 1.9 Stretch Ratio 1:9 0.9 0.15822 −8.4 2 Stretch Ratio 1:1 6.70.37989 0.0 2.6 Stretch Ratio 1:6 1.4 0.36274 −4.5 2.8 Stretch Ratio 1:80.9 0.25929 −31.7 3 Stretch Ratio 1:1 6.3 0.28287 0.0 3.4 Stretch Ratio1:4 1.8 0.30618 8.2 3.6 Stretch Ratio 1:6 1.4 0.2559 −9.5 3.7 StretchRatio 1:7 1.1 0.1804 −36.2 3.8 Stretch Ratio 1:8 0.8 0.12136 −57.1 4Stretch Ratio 1:1 7 0.2184 0.0 4.4 Stretch Ratio 1:4 1.7 0.19261 −11.84.6 Stretch Ratio 1:6 1.4 0.20104 −7.9 4.7 Stretch Ratio 1:7 1.1 0.1298−40.6 4.8 Stretch Ratio 1:8 1 0.1231 −43.6 5 Stretch Ratio 1:1 7.30.53874 0.0 5.5 Stretch Ratio 1:5 1.6 0.29392 −45.4 Note: The stretchratio numbers reflect an aim point; the film thickness values weremeasured.

INDUSTRIAL APPLICABILITY

The films of this invention are suitable for use in a wide variety ofpackaging applications.

The invention claimed is:
 1. A method for producing an orientedmulti-layer film comprising: A) co-extruding a multilayer filmcomprising: 1) a first layer prepared from a first high densitypolyethylene composition having a melt index, I₂, of from about 0.2 toabout 10 grams per 10 minutes, a density of from about 0.94 to about0.97 g/cc and from about 100 to about 9000 parts per million of anucleating agent, wherein the first layer is a core layer; and 2) asecond layer prepared from a second polyethylene composition having amelt index, I₂, of from about 0.2 to about 10 grams per 10 minutes and adensity which is lower than the density of said first high densitypolyethylene composition by an amount of from about 0.010 to about 0.060g/cc; and B) stretching said multilayer film in the machine direction ata stretch ratio of from about 1:2 to about 1:12; wherein melt index ismeasured according to ASTM D1238 at 190° C. using a 2.16 kg weight anddensity is measured according to ASTM D792.
 2. The method of claim 1,wherein said stretching is done at a temperature below about 120° C. 3.The method of claim 1, wherein said multilayer film is a three layerfilm having at least one skin layer prepared from said secondpolyethylene composition and a core layer prepared from said first highdensity polyethylene composition.
 4. The method of claim 1, wherein saidmultilayer film comprises at least five layers and wherein saidmultilayer film has two skin layers prepared from said secondpolyethylene composition and at least one core layer prepared from saidfirst high density polyethylene composition.
 5. The method of claim 1,wherein said multilayer film has a thickness of from about 0.5 to about3 mils after said stretching.
 6. A film made by the method of claim 1.7. A package prepared from a film according to claim
 6. 8. A method forproducing an oriented multilayer film comprising: A) co-extruding amultilayer film comprising: 1) a first layer prepared from a first highdensity polyethylene composition having a melt index, I₂, of from about0.2 to about 10 grams per 10 minutes; a density of from about 0.95 toabout 0.97 g/cc and a Mw/Mn of from about 5 to about 12, with theproviso that said first polyethylene composition is a blend of at leasttwo blend components comprising a first blend component having a Mw/Mnof from about 2 to about 4 and a second blend component having a Mw/Mnof from about 2 to about 4, wherein the first layer is a core layer; and2) a second layer prepared from a second polyethylene composition havinga melt index, 12, of from about 0.2 to about 10 grams per 10 minutes anda densizy which is lower than the density of said first polyethylenecomposition by an amount of from about 0.010 to about 0.060 g/cc; and B)stretching said multilayer film in the machine direction at a stretchratio of from about 1:2 to about 1:12; wherein melt index is measuredaccording to ASTM D1238 at 190° C. using a 2.16 kg weight and density ismeasured according to ASTM D792.
 9. The method of claim 8, wherein saidfirst blend component has a melt index I2 and said second blendcomponent has a melt index I₂′; wherein the ratio (I₂/I₂′) is greaterthan about
 10. 10. The method of claim 8, wherein said stretching isdone at a temperature below about 120° C.
 11. The method of claim 8,wherein said multilayer film is a three layer film having two skinlayers prepared from said second polyethylene composition and a corelayer prepared from said first high density polyethylene composition.12. The method of claim 8, wherein said multilayer film comprises atleast five layers and wherein said multilayer film has two skin layersprepared from said second polyethylene composition and at least one corelayer prepared from said first high density polyethylene composition.13. The method of claim 8, wherein said multilayer film has a thicknessof from about 0.5 to about 3 mils after said stretching.
 14. The methodof claim 8, wherein said first high density polyethylene compositionfurther contains from about 100 to about 2000 parts per million of anucleating agent.
 15. A film made by the method of claim
 8. 16. Apackage prepared from a film according to claim
 8. 17. The method ofclaim 1, wherein said nucleating agent comprises the calcium salt ofhexahydrophthalic acid.